A special problem arises in welding small-diameter pipes which cannot be rotated during welding, e.g. during installation of pipelines where it is necessary to ensure stability of welding quality with the achievement of high productivity. These pipes are welded nowadays mainly manually using coated electrodes, or welding is performed in shielding gases. The number of welded joints largely depends on the welder skill, quality of materials and welding conditions. A change in position of the welding zone on space during transition from the top point of the weld to the bottom point thereof complicates the process as it is necessary to correct welding conditions.
To avoid errors characteristic of the manual welding, microprocessor controlled welders are used in certain applications. This results in an increase in the number of welded joints and results in a certain improvement of productivity, the cost of welding equipment increases, and requirements imposed on skill of the operating personnel are more stringent, certain restrictions being imposed upon operating conditions of the equipment (e.g. ambient temperature, dust lading and humidity of air).
In certain applications a continuous flash butt welding is used which ensures stability of quality of welded joints with a high level of automation of the process. This welding method can be used both at factories and in the field. However, welded joints produced by this method are characterized by the presence of irregularly shaped flash having sharp projections and collars which forms on both outer and inner surfaces of the pipeline. The flash on the inner surface of the pipeline is to be removed as it reduces the cross-section thus causing resistance to fluids moving through the pipeline. The removal of such flash is generally carried out by means of cutting tools attached to a bar which is very difficult and can only be made at straight portions of the pipeline.
Methods of welding with an arc moving in magnetic field which are most promising and productive have recently come into use for welding small-diameter pipes.
Known in the art is a method for pressure welding with heating with an arc moving in magnetic field (DD, A, 129179), comprising placing parts being welded, e.g. pipes coaxially with each other with a working gap defined between their ends, a permanent magnetic field being created in the gap, and firing an electric arc which moves along the perimeter of the ends under the action of the permanent magnetic field. At the beginning of the process, the arc current is set up in such a manner that the ends of the parts being welded be heated to a temperature of plastic deformation, which is followed by a pulsed increase in the arc current and upsetting.
In the abovedescribed method, the time for heating the ends to the temperature of plastic deformation is set up in advance without taking into account eventual deviations of the arc current or dimensions of the parts being welded, e.g. diameter or wall thickness of the pipes being welded. This negatively affects stability of quality of welded joints. Such deviations are not taken into account either when a fixed value of the duration of a pulsed increase in the arc current is set up. In addition, the prior art method does not allow local changes in the gap caused by non-parallelism of the ends and their irregular shape to be determined. As a result, the resultant welded joint may have local defects.
Welding conditions, which are generally determined as a result of a number of trial weldings, should be varied in changing over for welding of parts of different size which involves much time and energy expeses.
Therefore, the prior art method for welding with heating with an arc moving in magnetic field cannot guarantee stability of quality of a welded joint.
Known in the art is a method for pressure welding of parts with heating with an arc moving in magnetic field (ZIS-Mitteilungen (DDR, No. 10, 1982, s.1051-1055), comprising placing parts being welded coaxially with each other with a working gap between their ends, an electric arc being fired in the gap, creating magnetic field in the gap, the electric arc moving along the perimeter of the ends of the parts being welded under the action of the magnetic field to heat the parts to a temperature of plastic deformation of the material of the parts being welded, determining the moment at which this temperature is reached, and beginning a pulsed increase in the electric arc current from this moment to a value at which temperature of the ends of the parts being welded reashes the melting point of the material of the parts being welded, with subsequent upsetting of the parts being welded.
In the prior art method, the moment of heating of the ends to the temperature of plastic deformation is determined by the amount of thermal energy stored in the ends of the parts being welded, e.g. pipes. The amount of thermal energy is, in turn, determined using a computer which measures current and voltage of the electric arc and carries out readings of the amount of thermal energy every 0.1 s.
It should be, however, noted that the amount of energy calculated on the basis of the measured current and voltage of the electric arc is practically the energy released from the arc. It is only a part of this energy that is stored in the ends of the pipes, and this part substantially depends on conditions of heat exchange with the environment (ambient temperature, presence of wind, eventual contact between the pipes being welded and structural members having a high heat conductivity, and the like) and is not always confident.
Therefore, one cannot judge on the amount of energy stored in the ends of the parts being welded by the calculated amount of energy.
In addition, the duration of the pulsed increase in current in the prior art method is set up in advance without taking into account eventual deviations of the degree of heating of the ends of the parts being welded.
Upon a change in size of the parts being welded, it is necessary to put in the computer the data corresponding to the new value of thermal energy after overpassing of which a pulsed increase in current should be carried out.
Inspite of the employment of a sophisticated electronic equipment in the system controlling the welding process, the method cannot ensure the measurement of a local amount of gap and cannot guarantee high stability of quality of welded joints. In addition, highly-skilled personnel should be employed for operation of the apparatus for carrying out this prior art welding method, and operating conditions are restructed to certain ambient temperature ranges and conditions of the environment.